Question: I know that it is not permissible to perform salat by employing the use of a loudspeaker or through the reflection of sound. However, big mosques were built in a manner that they could make the echo of sound stronger. How can it be permissible in this case?
ANSWER
The science that deals with sound is termed acoustics. To start with, let us see how the human voice is produced:

While one is speaking, vocal cords in the larynx tighten and become tense. The air exiting the lungs make vocal cords vibrate, with the result that voice is produced. Vibrating cords strike air molecules and make them vibrate, too. Then they make the other molecules next to them vibrate, and they reach until our ears. Thus, we hear sound. In air, sound travel in spheres in an orderly manner and in waves. Air itself does not travel. It transmits sound. In dry air, the speed of air is 340 meters per second. Water molecules transmit sound, too. The speed of sound in water is 1,500 meters per second. Solid objects transmit sound more rapidly. The speed of sound in steel and glass is 5,000 meters per second.

When sound waves that travel in air or in water bounce off a hard even surface, such as a wall or cliff, they change direction and come back from it. These waves that have returned form a second sound with the same qualities, which is called an echo.

The time difference between a sound's being heard and its echo's being heard is called the time of echo. It is possible to measure the depth of sea with the help of acoustics by making use of the reflection property of sound.

At places where the time of echo is above 0.1 second and which are far from a sound source, a reverberation and second and more echoes are produced. The sound that echoes back from mountains, desert, or other surfaces is not the actual voice of the human. For this reason, this second voice called an echo is not the speaker's actual voice coming out of his/her mouth. In order for acts of worship to be valid, not artificial voice but natural human voice is needed.

In a mosque, theatre, or conference hall, it is of great importance to transmit sound with the least echo and the highest clarity to the target audience.

Studies about acoustics date from Islamic architecture. They can mostly be seen in the Ottoman and Seljuk architecture as well. In mosques where thousands of people perform salat, physical qualities of an echo were calculated down to the smallest detail. All precautions were taken so that the voice of the imam could be heard from every corner.

It is written in encyclopedias that Mimar Sinan, while constructing the Suleymaniye Mosque, made calculations in a meticulous manner with the sound of bubbles forming in water that he boiled in the container of a hookah so that no echo would be produced. Mimar Sinan took precautions to prevent the formation of a second noise due to reflection of sound waves.

The purpose is not to amplify sound but to prevent echoes and to make natural sound be heard. It is written in fiqh books:

"When the imam’s voice does not reach everywhere, it is permissible for muadhdhins to repeat loudly so that the followers can hear, but shouting too loudly nullifies their own salats. Performing recitation too loudly resembles a worldly conversation. The imam’s reciting more loudly than necessary in salat does not nullify the salat, yet it is haram."

The sound emitted from a loudspeaker is neither the speaker's own voice nor an echo. It is a different sound similar to the speaker's own voice.

When one speaks into a microphone, one's voice is converted into electrical signals. As for a loudspeaker, it is a transducer that converts electrical signals into sound waves.